Bioavailability of cationic surfactants

Summary

By means of the Solid-phase micro extraction [SPME] technique it has been shown that the freely dissolved, bioavailable concentrations of cationic surfactants in the presence of natural sorbents, were significantly lower than nominal concentrations in various ecotoxicity test assays and in in-vitro cytotoxicity assays with cultured fish gill cells in the presence of serum. A soil sorption model for organic cations was developed.

Background

Cationic surfactants are known to be highly adsorptive to solid particles (e.g. sediment). The aim of this project was to evaluate the implications of sorption for the bioavailability of these compounds and to predict surfactant partitioning to sediments. The project is directly complementary to the ERASM PhD project “Bioavailability of surfactants in marine sediments”.

The research performed at the Institute for Risk Assessment Sciences [IRAS] at Utrecht University, was part of a larger project lead by the Helmholtz Centre for Environmental Research (UFZ), in Leipzig, funded by APAG . Whereas IRAS focused on a top-down approach, developing tools to measure and validate bioavailability of cationic surfactants , UFZ focused on a bottom-up approach, developing and validating sorption models for all organic cations.

Outcome of research

SPME was identified in the IRAS project as a simple technique to analyse the freely dissolved concentrations of cationic surfactants , which opens up many possibilities to perform environmental studies more accurately.

The results of the IRAS project revealed how the sorption affinity of quaternary ammonium surfactants to organic matter was influenced by organic matter type and medium composition. Sorption affinities to natural soils were predicted within a factor of ± 3. A suite of empirical rules were defined to facilitate detailed predictions of soil sorption affinities for strongly ionised bases and quaternary ammonium compounds.

The UFZ project developed and validated a soil sorption model for organic cations, based on two standard soil parameters - fOC and CEC -, and sorption data on two reference soil components (illite clay and pahokee peat). Customized HPLC columns, packed with a mixture of silicon carbide (SiC) and either individual soil components or natural soils, were identified as efficient and representative high-throughput tools to study a range of different aspects of the sorption processes of organic cations. A sorption dataset was created by the UFZ project for ~50 different organic cations on pahokee peat (as reference organic matter) and illite clay (as reference clay mineral) and two Eurosoil as natural soil standards. Combined with two standard soil parameters - fOC and CEC -, sorption data on these two reference soil components predict sorption affinities to natural soils within a factor of ± 3. Sorption of any organic cation, including cationic surfactants, was shown to be influenced by medium composition (salinity, divalent (e.g. Ca2+) concentration, pH), sorbent composition (e.g. CEC, fOC), and chemical structure (e.g. polarity, hydrophobicity).

Several electrostatic effects were well explained by NICA-Donnan model simulations that are well validated for metal sorption data. A suite of empirical rules were defined to facilitate detailed predictions of soil sorption affinities for strongly ionised bases and quaternary ammonium compounds.

The SPME tool showed that the freely dissolved, bioavailable concentrations of cationic surfactants in various ecotoxicity test assays - including acute toxicity tests with daphnid in the presence of humic acid, with oligochaete (Lumbriculus variegatus) in the presence of OECD soil, and in vitro cytotoxicity assays with cultured fish gill cells in the presence of serum, and toxicity tests with algae - were significantly lower than nominal concentrations, and normalised tests with different sorbent levels.